By a study of numerous reactions it is shown that: the rate of a reaction is directly proportional to the reactant concentrations, each concentration being raised to some power. A rate law or rate equation for a chemical reaction is an equation that relates the rate of a reaction to the concentrations or pressures of reactants raised to their powers. Thus for a substance A undergoing reaction,

A → B

The rate law or rate equation for this reaction is written as:

The proportionality constant k, called the rate constant, and n, called the order of the reaction.

Note that n is not stoichiometric coefficient in the balanced equation and it is determined experimentally. The order of a reaction can be positive or negative and can be an integer or a fraction.

Note two important points about rate law or rate equation:

The concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reaction does not contribute to the overall rate.

The value of the exponent n must be determined by experiment; it cannot be written from the balanced equation.